1. Field of the Invention
The present invention relates generally to lithographic techniques employed in the manufacture of integrated circuits (ICs), and, more particularly, to the fabrication of a reticle mask used in photolithography to process semiconductor wafers.
2. Description of the Related Art
A reticle mask, also referred to as a photomask, may be used to transfer a pattern to a semiconductor wafer. The pattern to be transferred onto the wafer is typically formed on a substantially transparent photomask substrate, such as quartz. Generally, standard photolithography processes are used to pattern a non-transparent material, such as a metal film over the photomask substrate. Chromium is a common material used to form the pattern.
FIG. 1A illustrates a commonly used binary mask 10 used to pattern a wafer. The binary mask 10 includes a substrate 12, such as quartz, on which chromium traces 14 have been formed to define a photomask pattern. Due to limitations imposed by the wavelength of light used to transfer the pattern, resolution at the edges of the patterns of the photomask degrades, thus limiting the application of the binary mask 10 as the geometry of the features to be formed on the wafer decreases.
FIG. 1B illustrates a prior art phase-shift mask 20, developed to increase the resolution of patterns that can be formed on a wafer. A phase-shifting region is formed by forming a trench 22 in the photo-mask substrate 12. A standard phase-shift mask 20 is generally formed by depositing chromium traces 24 of appropriate width and separation and etching the vertical trench 22 in the photo-mask substrate 12 in the region defined between adjacent traces 24. The essentially vertical walls (e.g., 85xc2x0 to 90xc2x0) of the trench 22 define phase edges 26 that provide a transition between high and low refractive index regions. The depth of the trench 22 determines the amount of phase shift produced by the phase-shift mask 20 relative to the wavelength of the incident radiation. Typically, the depth of the trench 22 is selected to provide a 180xc2x0 phase shift, and the width of the trench 22 is less than the wavelength of the incident radiation.
Exemplary techniques for forming the trench 22 are described in U.S. Pat. No. 5,308,722, entitled xe2x80x9cVOTING TECHNIQUE FOR THE MANUFACTURE OF DEFECT-FREE PRINTING PHASE SHIFT LITHOGRAPHY,xe2x80x9d and U.S. Pat. No. 5,851,704, ENTITLED xe2x80x9cMETHOD AND APPARATUS FOR THE FABRICATION OF SEMICONDUCTOR PHOTOMASK,xe2x80x9d both of which are incorporated herein by reference in their entireties.
The characteristics of the phase-shift mask 20 generally relate to a hard or strong phase-shift type mask, commonly known as an xe2x80x9calternating aperturexe2x80x9d or xe2x80x9cLevenson-typexe2x80x9d phase-shift mask. These types of masks include transmission regions (light transmitted through the substantially transparent regions) on either side of a patterned opaque feature (e.g., the chromium traces 24). One of these transmission regions is phase-shifted from the other (i.e., trench 22, and both sides transmit approximately 100% of the incident radiation. Light diffracted underneath the opaque regions from these phase-shifted regions cancels, thereby creating a more intense null, or xe2x80x9cdark area.xe2x80x9d The feature (e.g., polysilicon line or photoresist line) to be patterned on the wafer is defined by the null region.
FIG. 1C illustrates a chromeless phase-shift mask 30. Chromium traces 32 (shown in phantom) are used to define the trench 22 and are subsequently removed. The null region used to pattern the feature on the wafer forms below the phase edge 26. The chromeless phase-shift mask 30 is capable of patterning smaller features than the phase-shift mask 20 of FIG. 1B. In the chromeless phase-shift mask 30, the distance between adjacent features on the wafer, commonly referred to as the pitch, is defined by the width of the trench 22. The width of the trench 22 is determined by the spacing between the chromium traces 32, which are used in the reactive ion etch of the photo-mask substrate 12 to define the trench geometry.
Using present photolithography approaches, it is difficult to form the chromium traces 32 sufficiently close to pattern dense features on the wafer. A typical approach to forming the traces 32 involves forming the traces 32 larger than the desired size and using one or more isotropic etches to remove a portion of the material to arrive at traces 32 of the desired critical dimension. As is known to those of ordinary skill in the art, the subsequent etching of the traces 32 actually adds to the variability in the critical dimension, limiting the usefulness of this approach.
As is known to those of ordinary skill in the art, the ability of a photomask to imprint a pattern on a wafer is determined in part by the resolution and the depth of focus. Simplified equations for resolution and depth of focus are described below:                     R        =                                                            k                1                            ⁢              λ                                      NA              2                                ⁢                      xe2x80x83                    ⁢                      (            Resolution            )                                              (        1        )                                D        =                                                            k                21                            ⁢              λ                                      2              ·              NA                                ⁢                      xe2x80x83                    ⁢                      (                          Depth              ⁢                              xe2x80x83                            ⁢              of              ⁢                              xe2x80x83                            ⁢              Focus                        )                                              (        2        )            
The resolution and depth of focus depends mostly on the numerical aperture (NA) of the lens unit and the wavelength of the incident radiation. The correction factors k1 and k2 depend on the process, material, resist, etc. Other factors, such as chromatic and spherical aberrations in the lens used to project the light on the photomask, also have an effect on the resolution and depth of focus, but their relative contributions are small.
Another factor that theoretically affects the resolution and depth of focus is the partial coherence of the incident radiation. Partial coherence is a relative measure of the degree to which the incident radiation is columnated. For example, light from a laser is typically fully columnated (i.e., very little scattering; perpendicular angle of incidence), and is referred to as fully coherent (i.e., PC=1). On the other hand, light with a high amount of scattering (i.e., any angle of incidence), such as light that might come from a flashlight, is referred to as incoherent (PC=0). Light between these extremes is referred to as partially coherent. Typically binary photomasks are used with light having a partial coherence of about 0.65-0.7, and phase-shift masks are used with light having a partial coherence of 0.45-0.6. Due to the nearly vertical walls that define the phase edges of a phase-shift mask, changing the partial coherence of the light has essentially no effect on the resolution or depth of focus.
Previous methods used to increase resolution, and thus, decrease pitch, have involved decreasing the wavelength and/or increasing the numerical aperture. Both of these approaches increase resolution at the expense of depth of focus, and as of yet, have not been successful in implementing high-density feature layouts.
For the reasons described above, chromeless photomasks have seen little application in a production environment for extremely high feature densities. The present invention is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above.
One aspect of the present invention is seen in a method for forming a photomask. the method includes providing a transparent substrate and forming an opaque layer over at least a first portion of the transparent substrate. The opaque layer is patterned to define a mask pattern and expose at least a second portion of the transparent substrate. The second portion is etched to define a phase shifting region. The width of the phase shifting region defines a critical dimension. The critical dimension is measured, and the phase shifting region is etched based on the critical dimension to undercut the optically opaque layer.
Another aspect of the present invention is seen in a photomask including a transparent substrate and a phase shifting region defined in the transparent substrate. The phase shifting region includes sloped sidewalls having a slope of less than about 85xc2x0.